What Happens When We Smell?
The sense of smell is intimately connected to emotions and perceptions, which are also interconnected with pain.25-28 In fact, the neuroanatomy of olfaction possesses intricate reciprocal axonal connections with the primary emotion areas, which include the amygdala, hippocampus, and orbitofrontal cortex (OFC). How the brain determines and translates odors is extremely complex, individualized, and still not completely understood.25-31

One mechanism identified is through the brain's ability to map out odorant objects. Whereas prior research was focused mostly on the olfactory receptor neurons and olfactory bulb, recently a large focus has been on the central processing of an odor in the brain.25-31 This has led to a fascination with olfaction, with many implications in science.

Odor perception is first registered in the olfactory epithelium binding to the receptors of olfactory sensory neurons. These neurons then project to olfactory bulb glomeruli, synapse with the dendrites of mitral and tufted cells, are then conveyed to the lateral olfactory tract (LOT), and terminate in several brain areas in the frontal and medial temporal lobes.26,29 These areas are often referred to as the primary olfactory cortex and include the anterior olfactory nucleus, olfactory tubercle, piriform cortex, amygdala, and rostral entorhinal cortex.26,29-31

These areas then relay information to the OFC, agranular insula, hypothalamus, lateral and basolateral amygdala, perirhinal cortex, hippocampus, and striatum, which then link to areas associated with affective learning and memory. Interestingly, the sense of smell is the only perception that occurs without a thalamic relay between the subcortical areas to the prefrontal cortex.26,29-31 It is, however, believed that the piriform cortex may behave as a sensory association cortex, such that it modulates the identification, categorization, and discrimination of olfactory stimuli.32-35

It has been found that individuals with focal orbitofrontal lesions can mostly still detect odor but have difficulty in odor discrimination, identification, and memory association. This indicates a disruption in the local and central processing of smell.28 The limbic connections of olfaction explain the mediation of physiological, emotion, and behavioral responses to olfactory stimuli.25-31 These networks are bidirectional, indicating the importance of brain interpretation of odorants and communication for appropriate behavioral responses.25-34

The Amygdala Connection to Pain
The limbic structure most often linked to the emotional aspects of olfaction perception is the amygdala, which is also an important mediator for pain reception and behavioral responses. 25-31,36,37-39 Olfactory stimulation has been reported to directly activate amygdala neurons, bypassing the primary olfactory cortex, before arriving at the secondary (association) olfactory cortex situated in the middle of the OFC.25 This direct connection may be important for survival due to the urgency of interrupting pain reception and danger signals. Therefore, it's important to first understand the processing of pain in this brain region to comprehend the connections between olfaction, pain, and emotions.

The amygdala is a group of several nuclei located in the temporal lobe. These include the superficial nuclei and the lateral (LA), basolateral (BLA), and central nuclei (CeA), with the latter three being most prominent in relationship to sensory processing.36-39 The LA receives input on sensory information, including nociception, from nuclei of the thalamus and cortical areas.38 It attaches the sensory information with affective content through associative processing with the BLA.37 This is then transmitted to the CeA for modulating pain behavior which sends signals through descending pain pathways. It is believed that the affect-related information from the CeA generates the emotional response to pain.37 The laterocapsular division of the CeA (CeLC) is also a direct receiver of pain information (i.e., not processed through the thalamus or cortex) from the parabrachial area and through the spino-parabrachio-amygdaloid pain pathway.38

Beyond its sensory processing, the amygdala also affects pain perception related to its role in the extrahypothalamic component of the stress response.40-42 It contains corticotropin-releasing factor (CRF) receptors within its structures, and CRF has evidence of exhibiting antinociception at the level of the brain, spinal cord, and peripheral sensory neurons.40-45

Due to the fact that CRF receptors 1 and 2 differentially affect pain sensitization in the brain, dysregulation in their signaling could lead to variances in pain perception.42,43 Results from a literature review in the European Journal of Pain postulated that neuropathic pain may result from CRF receptor signaling disturbances in the amygdala. Specifically, endogenous CRF or blocking of CRF1 receptors could lead to hyperalgesia. Therefore, it has been suggested that the amygdala may serve as a "hot spot" in supraspinal descending pain control, switching on and off chronic pain.43

Furthermore, it has been shown that corticotrophin releasing factor (CRF) upregulated in the limbic structures is associated with neuropathic pain as well as mood disorders, such as anxiety and depression. This is likely to be independent of hypothalamic-pituitary-adrenal (HPA) axis stimulation.44 Therefore, olfaction could potentially impact central pain response through modulation of physiological feedback (which will be discussed later in this article).

CRF receptors in the amygdala may also be linked to pain reception in the central nervous system (CNS) through inflammatory signaling reception of CRH. A 2006 rodent study stated, "The stress neuropeptide CRH may regulate neuroinflammation by inducing the apoptosis of microglia, the major cellular source of inflammatory mediators in the CNS."45

The Smell of Pain in Rodents
The neurological and physiological connections between the processing centers involved in olfaction, emotions, and pain suggest that the sense of smell has multimodal effects. This is exemplified by some interesting rodent studies that examine the impact of odor and its interaction with fear, the amygdala, and epigenetic programming in pain perception and behavioral response.

A 2014 study in the Proceedings of National Academy of Science examined the conditioned emotional response evoked by a smell using a mouse model of social transmission of fear. In the study's first phase, mother rodents were conditionally trained to fear the odor of peppermint through simultaneous receiving small electric shocks. This was meant to cause an olfactory association of peppermint to pain.

The second phase was to determine the impact of the mother's fearful response to the conditioned smell on her pups' future behavior. In the experimental group, the mothers were conditioned and the odor was presented to her in the presence of her offspring, without her receiving the shock. There were also two control group mothers. One group consisted of mothers that were fear conditioned with shock but not the peppermint stimulus. As witnessed by her offspring and the researchers, these scared mothers also displayed some unease when placed back in the same environment where the shock was received. The second control group consisted of peppermint-exposed mother rats that had not received the shock. These mothers were later exposed to the same mint smell in the presence of their groups of pups.

After 7 days, the pups' responses to what they experienced were tested in a Y-maze. Overall, the pups' responses demonstrated social transmission of fear by olfaction and a conditioned stimulus. Specifically, the tiny creatures that had observed their mothers that were conditioned and reexposed to the peppermint odor had a significant freeze response to the peppermint stimulus.

Following the test, the pups were exposed to substitute mothers, in order to rule out that their response to the stimulant odor was not related to behavioral differences from rearing by a fearful mother. The substitute mother was either one who was previously frightened with the peppermint odor or within the two control conditions previously stated. The pups with the scare-by-shocks and fearful-of -peppermint mothers also showed aversion when peppermint odor was presented. This lasted throughout rodent adolescence without generalizing to a novel odor.

When the authors tested for neural activity in the brain related to the response of the rodents (using c-Fos early gene expression and C 2-deoxyglucose autoradiography), the basal and lateral amygdala nuclei were active.14 The Grueneberg ganglion (GG), an olfactory subsystem, was needed in order to transfer the conditioned fearful smell to the pups, as evidenced by axotomy.

The results demonstrated that maternal fear expression, caused an induction of the amygdala by increasing corticosterone (CORT), induced cue specific learning. Furthermore, a specific level of CORT was needed in order to ignite a fearful response. It was determined that experiencing the mother's fearful response was sufficient to raise their pups' CORT and support amygdala plasticity for the behavioral response of her offspring. The authors of the study also demonstrated that epigenetic and learned social transmission of fear could be suppressed by blocking amygdala activation, indicating the critical role of this brain area. Finally, and most relevant to this article, the researchers supported that olfaction can serve as a stimulus for epigenetic programming of emotions and behavioral responses.46

Another fear-odor association study in rodents that was published in Nature Neuroscience provided more evidence of this olfaction-emotional connection.47,48 Specifically, the epigenetic inheritance of fear in laboratory mice was tested by using the smell of acetophenone, a chemical scent comparable to cherries and almonds. The researchers distributed the scent around a small chamber while simultaneously giving small electric shocks to male mice. Soon the researchers observed that the rodents would shudder in the presence of the odor, even without a shock, due to associative learning.

One fascinating aspect of this study was that despite never being exposed to this smell, the offspring of the sensitized male mice exhibited increased sensitivity when they were introduced to the chemical scent. Interestingly, the third generation of the mice exposed to the odor, as well as those conceived in vitro, also inherited this same fearful reaction as their forebears. This demonstrated that an olfactory stimulus may, in fact, be enough of an emotional stimulus to modulate pain reactions in mice through epigenetic transfer.

Interestingly, these epigenetic transmissions were associated with physiological and neurological changes. Specifically, the responses of the fearful rodents were found to correlate to changes in the brain structures associated with the processing of odors. The researchers found that there were greater numbers of neurons that produced a receptor protein for this odor in the acetophenone-sensitized mice and their decedents than in the control mice. The structures to the acetophenone-detecting receptors which send signals to different parts of the brain (such as those associated with fear) were also larger.47

In an article in Nature that reported on the study, the researchers suggested that epigenetics and methylation were at play, "In the fearful mice, the acetophenone-sensing gene of sperm cells had fewer methylation marks, which could have led to greater expression of the odorant-receptor gene during development." The connection of how smell links to pain transgenerationally was a mystery, but it was suggested that odorant receptor proteins were present on the sperm cells. Another theory was that some odorants have physiological effects via their dispersion in the bloodstream similar to microRNAs that control genes.47

These epigenetic behavioral and physiological transmissions of emotions related to odors is not just displayed in the four-legged. The connection between emotional trauma, olfaction, and physiological responses has also been validated in humans.